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Some critics estimate that up to 100 Tory MPs may vote against the idea and the move by the London mayor, the education secretary and a dozen other senior figures will only widen the rift between those in favour and those against.

Under the proposals, due to be detailed this week, churches and other venues will be allowed to "opt in" to holding civil marriage ceremonies.

Ministers will offer a guarantee that no institution will be forced to marry gay people on their premises - but some Tory MPs and religious groups have questioned whether it would stand up to challenges under the Human Rights Act.

'Civil partnerships are not marriages, which express a particular and universally understood commitment.'

"If there is any church or any synagogue or any mosque that does not want to have a gay marriage, it will not - it absolutely must not - be forced to hold it," David Cameron said. "This is a free vote for members of parliament but personally I will be supporting it."

While the move to modernise the centuries-old institution of marriage may have popular support, Mr Cameron risks an internal divide. Tory MP Peter Bone claims several cabinet ministers will vote against the move and that the Conservatives are split 50/50 on the issue.

"Despite the PM's assurance, the redefinition of marriage - because of the European Convention on Human Rights - will force churches to marry same-sex couples. This will outrage millions of people and hugely damage the government in electoral terms," Mr Bone told Channel 4 News.

"As it is a free vote, undoubtedly cabinet ministers who have expressed concerns about the redefinition of marriage will vote according to their conscience," Mr Bone said. "As for the exact split in Conservative votes, it is only an estimate, but there are clearly many Conservative MPs on each side of the argument."

'Most parents don't want their children to be gay'

Tory backbencher David TC Davies has expressed his concerns over gay marriage and suggested most parents did not want their children to be gay.

"If there are any sort of areas where there isn't full equality with married couples then I'd be more than happy to support making changes to civic ceremonies, so I really don't know why we need to go ahead with this at all," the Monmouth MP told BBC Radio Wales.

"I think most people are very tolerant and have no problem at all if people are gay but, and I hate to say this in a way because I expect it's going to cause controversy, but I think most parents would prefer their children not to be gay, knowing most parents want grandchildren if nothing else."

Mr Davies said he was not sure the government could guarantee churches would not be forced to hold gay marriage ceremonies given the power of European courts.

"What I'm concerned about is what we were originally given a consultation on, and that is having gay marriage recognised by law which opens to door to all churches being forced to do that," Mr Davies said.

"It changes the way that sex education is going to be taught in schools.

"It's going to have an impact on teachers and I think it goes against what a lot of people feel very strongly about, particularly within the Conservative Party."

Gay weddings in 2014?

Legislation is expected to be introduced early next year and could mean the first gay wedding ceremonies would be held in the spring of 2014.

The new Conservative group has been put together by Nick Herbert, the former police minister who resigned from government in September's reshuffle.

As well as Boris Johnson and Michael Gove, it includes Transport Secretary Patrick McLoughlin, who is Catholic, Foreign Office Minister Alistair Burt, an evangelical Christian, former Tory Ministers Lord Fowler and Nicholas Soames and Scottish Conservatives leader Ruth Davidson.

In a letter to The Sunday Telegraph announcing the formation of the group, the 19 declared:

"Marriage should be open to all, regardless of sexuality.

"We recognise that civil partnerships were an important step forward in giving legal recognition to same sex couples.

"But civil partnerships are not marriages, which express a particular and universally understood commitment."

Coalition leaders have previously indicated they support the idea of equality reform, including allowing homosexual partners to have the same civil marriage rights as heterosexual couples and marry in the church or religious setting of their choice.